Light control means



Aug. 22, 1944. J. R. BAL LE'Y 2,356,195

LIGHT CONTROL MEANS Filed May 25, 1941 2 Sheets-Sheet 1 INVENTOR. JAMES R. BAL SLE Y Aug. 22, 1944.

J. R. BALSLEY 2,356,195 LIGHT CONTROL MEANS Filed May 23. 1941 2 Sheets-Sheet 2 INVENTOR. JAMES R. BALS LEY Patented Aug. 22, 1944 LIGHT CONTROL MEANS James R. Balsley, Stamford, Conn., assignor to Incorporated, Stamford. Conn., a corporation oi! Connecticut Application May 23, 1941, Serial No.-394,903

12 Claims. (01. 315 151) James R. Balsley',

The present invention relates to means for controlling and stabilizing electrically energized light sources, and particularly to means for stabilizing such light sources through the use of what may be termed optical feedback.

- In my prior patent, No. 2,242,638, issued May 20, 1941, I disclose a means for maintaining a -constant light intensity utilizing an electron discharge tubein series with a light source, the impedance of said tube being varied by the voltage from a photoelectric cell receiving its excitation from the controlled light source.

The present invention employs the same optical feedback principle as disclosed in the aforesaid patent, but in this instance the controlling electron discharge tube is in shunt with the light source rather than in series with it. The present invention thereafter acts to achieve the same result as the prior one, but does this in a difierent mode which is in some instances preferable.

It is an object of the invention to provide a means for controlling and stabilizing electrically energized light sources utilizing an electron discharge tube in shunt with the controlled light source.

It is a further object or the invention to pro vide such a circuit which is simple and economical to install.

Further objects and features of the invention will become clear when the following description is considered in connection with the appended drawings, in which Figure 1 is a diagram of a circuit for maintaining constant intensity of a light source utilizing an electron discharge tube in shunt with the source;

Figure 2 is a diagram oi! a circuit similar to Figure 1, in which, however, only a portion of the current to the light source is supplied by an electron-{discharge tube,.the greater portion being supplied by a generator;

Figure 3 is a diagram similar to that of Figure 1, illustrating the modulation of a light source in by a negative potential is such a manner that its intensity is varied in accordance with the modulation only without any foreign disturbances; and

Figure 4 is a diagram showing the application of the circuit of Figure 1 to the problem of copying photographs, documents, and the like, making an exact copy as regards density and contrast or giving any desired variation of either or both irrespective of the latent contrast of the copy paper or film being used.

Referring to the drawings and especially to Figure 1, there is shown at H a battery or other current source which suppliespower to the lamp i and to the control unit. In series with the battery is a voltage dropping resistance 12. Connected between the low potential side of resistance l2 and the negative terminal of current source II is an electron discharge tube l3, said electron discharge tube being shown as a triode,

that is, a tube having anode, cathode and control electrode.

Likewise connected between the low poten tial side of resistance l2 and the negative terminal of battery II, is a second electron discharge tube H, in series with which is a resistance 15.

Tube I4 is also a triode, having an anode, cathode, and grid, the anode of tube l4 being connected through a battery I 6 to the grid of tube l3 whereput on the grid of tube I3. I

Alsoconnected from the low potential side or resistance l2 to the negative terminal of battery I II, is a photoelectric cell H, in series with a re-.-

sistance l8, said resistance being a coupling resistance.

which leads through a battery 20 to'the' grid of tube l4.

Lamp 2| which is the light source to be controlled, is likewise connected from the low potential side of resistance l2 to the negative terminal of battery II, this lamp being arrangedso that a portion of the light therefrom reaches the a change in ambient temperature, this will bring about a reduction of the resistance of cell I|,-

thus causing the grid of tube It to become more negative. This, in turn, reduces the voltage drop across resistance l5, causing the grid of tube l3 to become more positive, thus increasing the total current through resistance l2 and, of course, reducing the voltage across lamp 2|. In a similar manner, if the light intensity tends to decrease, the resulting decrease in current through resistance l2 will compensate for this tendency,

In the event that a constant current source is Intermediate the coupling resistance HI and the photoelectric cell I1 is a connection l4 arev available, it is obvious that resistance l2 may be eliminated, since the photoelectric cell will then control the current flow through tube l3, which is one of the two major paths available for the current, the other being the lamp itself.

For uses where the disturbances which tend to alter the intensity represent depths of modulation which are appreciably less than one hundred per cent, that portion of the current which will be considered as the "steady current may be supplied by a battery or by a generator. The portion of current required to compensate for disturbances may be regulated by the use of an electron discharge tube as has been previously described. Figure 2 shows a circuit utilizing a generator for the purpose mentioned, and also for performing another function, that of compensating automatically for slow variations in intensity even of great amplitude. The generator is .represented in Figure 2 as having an armature 30 and field windings 3| and 32, the field 3| being so connected that as the current through it increases, the output voltage of the generator is reduced. The circuit components I3, l4, |5, i5 and 2| function exactly as described above in connection with Figure 1 for all light variations which occur at frequencies higher than a few per second.

For slow changes they have an additional function that of altering the output of the generator through automatic variations of the field strength. These variations will take place at frequencies up to a limit which is determined by the characteristics of the generator itself.

This type of circuit has practical application for such operations as the printing of motion picture film where it is imperative that the average value of light intensity be kept constant and where rapid variations of intensity of as little as a few per cent may ruin the print. It may also be used to compensate for variations in intensity normal for any carbon arc.

\ InFigure 3 there is shown a circuit which is generally similar to that of Figure 1, the circuit of Figure 3 being particularly designed, however, to permit of modulation of a light source. Like parts in Figure 3 have been given the same reference characters as in Figure 1 for the purpose of convenience. In connection with Figure 3, it may be stated that this arrangement permits the use of a mercury vapor lamp, which has not heretofore been possible except in a series arrangement as shown in my Reissue Patent No. 21,907. In the past, low pressure gas discharge tubes have been utilized for modulation purposes, as for example i'or recordingsound on film, but these tubes were not satisfactory due to insufficient light intensity and non-linear characteristics. When mercury was added to such lamps or used alone, the intensity was increased to the, desired value but other difficulties arose. Used in this manner, non-uniform characteristics were produced which were difficult or impossible to compensate. The response was consequently deficient in high frequencies, distortion was high, and, in addition, the average intensity could not be controlled within sufficiently close limits to meet modern requirements. Incandescent lamps have likewise been utilized for the purposes of recording and other uses involving modulation but were unsatisfactory since the thermal inertia of the filaments was too great for practical usage at high frequencies.

In making photographic sound records it is customary to focus the light source on a narrow slit and to focus the image of the slit on the film. Unless the light source, lenses, cell, and all other parts of the system are very firmly fixed, vibration occurs which creates a variation in light intensity on the film which is eventually reproduced as so called microphonic noise. The same result obtains if the filament of an incandescent lamp vibrates or the arc in a gas discharge tube does not remain in constant position. All of the above are disadvantages which are extremely difficult to overcome by mechanical means. If a circuit such as that of Figure 3 is utilized and means provided to illuminate the photo-electric cell H from a point between the last of the disturbing elements and the film, all disturbances, whether arising from the lamp itself or from any of the elements between the lamp and the film, will be compensated.

In the circuit of Figure 3, current is supplied from the source I to the electron discharge tube I3 and the lamp 2| in shunt in the same manner as described in connection with Figure 1. A simple three-element tube 14 of Figure 1 is replaced in the circuit of Figure 3, for the purpose of cir cuit simplification, by a multi-element tube ll containing two control grids. The plate of this tube is connected in a circuit in the same manner as theplate of tube H of Figure 1. Tube 00 has a screen grid which is connected through a batprovides a negativeor inverse optical feedback from the lamp 2| to the input. In brief, a fractional part of the output voltage here designated as ,8 is fed back to the input of the amplifier. Black, in Patent No. 2,102,671, has shown that with an amplifier having a gain of when #18 is large as compared with unity, the relation of output voltage to the input voltage is proportional to and is independent of all other factors. In the case illustrated, the light intensity is proportional to since the feedback voltage, that is, the voltage developed across resistance I8, is a function of the light intensity. It, therefore, follows that the light intensity will be proportional to the modulation voltage at all frequencies where the requirements of feedback and gain are met, and that the light intensity will be independent of all other factors, such as supply voltage changes, ageing of the lamp, mercury sputtering, etc.

Figure 3 illustrates only one of the many circuits which may be employed utilizing the advantages of optical feedback for such purposes as sound recording, signalling, and television. It will be obvious that electrical inverse feedback means alone, whileeffective for correcting defects within the amplifier, are entirely ineffective for corrections external to that amplifier and that the means of Figure 3 corrects not only for defects arising within the amplifier. but also for defects arising between the point in the system where the feedback voltage is applied and the point where the useful work is done by the beam of light.

Figure 4 is a diagram of a circuit which may be used for facsimile reproduction, In this circuit, the circuit of Figure 1 is modified by placing a second photo-electric cell therein, this cell being located between one terminal of the lamp 2| and the resistance ll.

In operating this circuit for copying, as for example copying a document which is faded or v yellowed and which is to be copied with substantial contrast between black andwhite, the document to be copied is scanned by well known means by a spot of light which will be reflected from the scanned surface onto the photo-electric cell 33. Light from the tube II is used to scan the surface of the, light sensitive paper. In the circuit, as shown in Figure 4, cell I'I, cell 33 and resistance Ill, together determine the grid bias oi tube I4 and consequently the light intensity of lamp 2|. Light from the lamp ii is caused to fall on cell II in the same manner as has been described with reference to Figure 1. Then by making proper adjustments a copy may be produced which will be in desired contrast, it being obvious. that by altering the connections the copy may beeither a positive or negative copy of the original. Details of such adjustments are not given here since they are exactly similar to those mentioned in my Reissue Patent No. 21,907.

While I have described various embodiments of my invention, it is obvious that changes may be made in details of the circuits without in any way departing from the spirit of the invention. Therefore I do not'wish to be limited in any way by the foregoing description, but on the contrary wish my invention to be limited only v the appended claims.

What is claimed is:

1. The method of controlling light intensity which comprises energizing a light source in shunt with an electron discharge tube, said tube having a fixed grid bias, causing a portion of the light from said light source to fall upon a photoelectric cell and causing all components of the output of said photoelectric cell to control the plate current of said tube.

2. The method of controlling light intensity which comprises energizing a light source in shunt with an electron discharge tube, said tube having an anode, a cathode and a control grid, causing a portion of the light from said light source to fall upon a photoelectric cell, causing all components of the output of said cell to be impressed upon the input of an amplifier, said amplifier being in series with a source of fixed voltage, and utilizing the outputof said amplifier to control the grid voltage of said electron discharge tube.

3. The method of controlling light intensity which comprises energizing a light source in shunt with an electron discharge tube, permitting a portion of the light from said light source to fall upon a photoelectric cell, causing all components of the output of said photoelectric cell to regulate the current fiowing through an amplifier tube having a fixed grid bias, and-, permittin the changed current flow through said-amplifier tube to effect a change in voltage on the grid of the electron discharge tube to thereby" control the energization of the light source.

4. The method of modulating a light source and simultaneously controlling the average intensity thereof, which comprises energizing the light source in shunt with an amplifier, providing the amplifier with excess gain over that required, converting a portion of the light to voltage by photoelectric means, and feeding back all components of said voltage to the input of the amplifier with such polarity of direct current components and such phase of alternating current components that the ampli er gain is reduced to the required value for all components.

5. In a system for controlling the average intensity and modulation of a light source, an amplifier having an input section and an output section, means, for supplying a fixed bias and a modulation voltage to the input section of said, amplifier, a lamp connected in shunt with the output section of said amplifier, photoelectric means for converting a portion 01 the light emitted by said lamp to voltage, and means for applying said voltage to sa d input section whereby the gain of the amplifier is reduced.

6. In a light modulatingsystem, the 'combinasaid second electron, discharge tube, means to impress 'all components of the voltage generated in said photoelectric cell on the grid of said second electron discharge tube, means to impress modulation voltage on the grid of said second electron discharge tube, and means in the plate circuit of said second electron discharge tube and connected to the control electrode of said first electron discharge tube to cause the changes in voltage on the control electrode of the second tube to be elIective to cause corresponding changes on the grid of said first tube.

7. In a device for controlling light intensity, in combination, a light source, an electron discharge tube connected in parallel with said light source and takin energy therefrom, said tube having a fixed grid bias, and a photoelectric cell mounted in position whereby light from said source falls on said cell, all components of the output current from said cellbeing effective to alter the voltage on the control electrode of said tube.

8, In a device for controlling light intensity, the combination of an electron discharge tube having an anode, a cathode and a control grid, alight source connected in a ,paralleicircuit with said electron discharge tubefla photoelectric cell in position to receive light from said light source, a second electron discharge tube having a fixed bias on the grid thereof, means to apply the voltage generated in said photoelectric cell to the grid of said second tube, and means comprising an impedance in the plate circuit of said second tube and connected to the control electrode of said first tube whereby the change in voltage on the grid of said second tube is effective to cause a corresponding change in the opposite direction on the grid of said first tube.

9. In a device for controlling light intensity, an electron discharge tube, a generator, a light sourceand said electron discharge tube in paralerator in accordance with the variation of plate current in said tube.

10. In a device for controlling the intensity of a light source, the combination of an electron discharge tube having an anode, cathode and control grid, 9. light source connected in parallel to said electron discharge tube, a photoelectric cell in position to receive light from said source, a second electron discharge tube comprising a cathode, anode and a plurality of control electrodes, means to apply the voltage generated in said photoelectric cell to one of the control electrodes of said second tube, said one of said control electrodes having a fixed bias, means to apply modulation voltages to a second control electrode of said second electron discharge tube, and means in the plate circuit of said second electron discharge tube and connected to the control electrode of said first tube whereby the change in voltage on the control electrodes of the second tube are effective to cause corresponding changes on the grid of said second tube.

11. In a device for copying and facsimile reproduction, in combination, an amplifier, a current source, alight source in shunt with said amplifier, both being energized from said current source, a light sensitive cell positioned to receive light from said light source, a second light sensitive cell adapted to receive light from a document to be copied, and means under the Joint control of said first and second light sensitive cells for controlling the gain of said amplifier.

12. In a device for copying and facsimile reproduction, in combination, an amplifier, a current source, a light source in paralled with said amplifier, both being energized from said current source, a light sensitive cell positioned to receive light from said light source, a second light sensitive cell adapted to receive light from a document to be copied, means under the joint control of said first and second light sensitive cells for controlling the gain of said amplifier, and means to adjust the joint control means to vary the-control of the amplifier gain.

' JAMES R. BALSIEY. 

